The present application pertains to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging with insertable gradient coils and will be described with particular reference thereto. It is to be appreciated that the invention will also find application in spectroscopy and other processes and apparatus in which accurate and predictable magnetic field gradient and resonance responses are sought.
Commonly, magnetic resonance imaging systems have a built-in, whole body gradient coil surrounding the patient receiving bores. For maximum efficiency, it would be advantageous to use gradient coils of the minimum diameter or size which will accept the portion of the subject to be imaged while maintaining gradient linearity. Because energy for the gradient field varies roughly with a fifth power of diameter in free space, reducing gradient coil size has a dramatic beneficial effect on power consumption. The larger diameter coils further have larger inductances which slow the switching speed of the gradient magnetic fields.
To achieve these advantages and others when imaging smaller regions of the patient, insertable gradient coils are often inserted into the bore of the magnetic resonance imaging system. These insertable coils include head coils, surface coils, biplanar gradient coils, and other special purpose gradient coils that are receivable in the main bore of the magnet assembly.
During an imaging sequence, a series of radio frequency pulses are applied in coordination with magnetic field gradients. The radio frequency pulses are typically applied by a whole body radio frequency coil which also surrounds the main magnet bore into which the insertable gradient coil has been inserted. The inserted gradient coil applies gradient magnetic field pulses for phase encoding, frequency encoding, slice selection, and the like of a limited region of the subject within its smaller bore. The magnetic resonance signals emanating from the region within the inserted gradient field coil are received, typically using a dedicated local RF coil, and processed to generate image representations.
The examined subject extends beyond the inserted gradient coil into regions of the main bore to which the whole body RF coil and, to a lesser extent, the local RF coil are sensitive. One problem with insertable gradient coils is that they generate magnetic fields which extend beyond the bore. In particular, the generated magnetic field gradients are linear through a target region in the insertable gradient coil bore reaching a maximum at a roll-off point near the edge of the insertable gradient coil. Past the roll-off point, the gradient field strength approaches zero with distance from the insertable gradient field coil. Thus, portions of the subject within the sensitive field of the whole body RF coil and to a lesser extent the local RF coil but outside of the inserted gradient coil are subject to gradients of the same strength as portions of the patient within the inserted gradient coil. Signals produced in regions with the same strength gradient magnetic field are indistinguishable by conventional magnetic resonance equipment and reconstruction algorithms. Hence, there is confusion of signals from the external regions with those from the target region within the inserted gradient coil. This confusion results in an alias image of material outside of the insertable gradient coil superimposed on the desired image from the target region within the inserted gradient coil.
one technique for overcoming this problem is to design the gradient coil such that its roll-over points extend past the limits of the target subject, or at least far enough away from the local RF receiver coils that the signals from these regions will not be strongly coupled to the RF coil. Unfortunately, extending the gradient coil roll-over points or limiting the length or extent of the RF coil each have significant performance costs for a magnetic resonance imaging system.
Another technique for preventing alias artifacts from portions of the subject beyond the roll-off points is to shroud those portions of the subject in an RF shield. Although the RF shield limits the aliasing artifacts, it has several drawbacks. First, the presence of the RF shield affects the performance of the RF coil in a detrimental fashion. Second, access is limited to the shielded region. Further, sometimes the reject region of the subject to be shielded is not amenable to the placement of an RF shield. Placing the RF shield too close to the inserted gradient coil affects the magnetic resonance in the target region within the inserted coil.
The present invention contemplates a new and improved technique for eliminating the aliasing in small gradient coils.